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Almost all adult arachnids have eight legs, and arachnids may be easily distinguished from insects by this fact, since insects have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae, serve in feeding and defense. The next pair of appendages, the pedipalps have been adapted for feeding, locomotion, and/or reproductive functions.

Like all arthropods, arachnids have an exoskeleton. They also have an internal structure of cartilage-like tissue, to which certain muscle groups are attached.[4]

There are some adaptations for life on land. They have internal respiratory surfaces. These may be trachea (tubes), or a modification of gills into a 'book lung'. This is an internal series of lamellae used for gas exchange with the air.

Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Only the harvestmen and some mites eat solid food particles. Predigestion avoids exposure to internal parasites.[5] Several groups secrete venom from specialized glands to kill prey or enemies. Several mites are external parasites, and some of them are carriers of disease (vectors).

Arachnids pour digestive juices produced in their stomachs over their prey after killing it with their pedipalps and chelicerae. The digestive juices rapidly turn the prey into a broth of nutrients which the arachnid sucks into a pre-buccal cavity located immediately in front of the mouth. Behind the mouth is a muscular, pharynx, which acts as a pump, sucking the food through the mouth and on into the oesophagus and stomach. In some arachnids, the oesophagus also acts as an additional pump.

The word Arachnida comes from the Greek for 'spider'. In legend, a girl called Arachne was turned into a spider by the goddessAthena. Arachne said she'd win a weavingcontest against the goddess. Arachne won, but Athena became angry, turning her into a spider for her hubris (pride) in challenging a goddess.